from article called "boot with grub"

The floppy can then be unmounted, umount /floppy, but leave it in the drive. The GRUB floppy is prepared and ready for the final installation, which is to install the GRUB boot loader in the MBR of the floppy itself. For that, we will use the grub executable we have built with our Linux installation. Start the executable at the Linux command prompt: grub.

This brings up an emulator of GRUB's command shell environment, which looks like Figure 1. We will discuss the features of this shell in more detail a little further on. For now, enter the following series of commands at the grub prompt:


grub> root (fd0)
grub> setup (fd0)
grub> quit

And that's it! This sequence of commands completes the installation of GRUB on the floppy disk. It is now bootable and will allow us to boot any other OS on our system.

My question:
If we have umount the floppy how do the commands "root (fdo)", "setup (fdo)" get written to the floppy?